1. Field
This invention relates in general to nuclear power plants, and more particularly, to passively activated apparatus for providing auxiliary power to safety equipment in a nuclear power plant under emergency shutdown conditions where there is a loss of conventional onsite and offsite electrical power.
2. Related Art
A nuclear reactor, such as a pressurized water reactor, circulates coolant at high pressure through a cooling circuit traversing a reactor pressure vessel containing nuclear fuel for heating the coolant and a steam generator operable to extract energy from the coolant for useful work. A residual heat removal system is typically provided to remove decay heat from the pressure vessel during shutdown. In the event of a loss of coolant, means are provided for adding additional coolant to the system. A coolant loss may involve only a small quantity, whereby additional coolant can be injected from a relatively small high pressure makeup water supply, without depressurizing the reactor coolant circuit. If a major loss of coolant occurs, it is necessary to add coolant from a low pressure supply containing a large quantity of the coolant. Since it is difficult using pumps to overcome the substantial operating pressure of the reactor coolant circuit, e.g., 2250 psi or 150 bar, the reactor coolant circuit is depressurized in the event of a major loss of coolant so that coolant water can be added from an in-containment refueling water storage tank at the ambient pressure within the nuclear reactor system containment shell.
The primary circuit of an AP1000® nuclear reactor system 22 (shown in FIG. 1), offered by the Westinghouse Electric Company LLC, Cranberry Township, Pa., of which the present invention is a part, uses a staged pressure reduction system for depressurizing the primary coolant circuit, which is illustrated in FIGS. 1 and 2. A series of valves 72 couple the reactor outlet 56 (also known as the “hot leg” of the primary coolant circuit) to the inside of the containment shell 54 through spargers 74 in the in-containment refueling water storage tank 50, which vent and dissipate the energy of the hot leg coolant into the refueling water in the tank. When the tank heats up and emits steam, the steam is condensed on the inside of the containment shell. When initially commencing the depressurization, the coolant circuit 46 and the in-containment refueling water storage tank are coupled by the depressurization valves 72 through one or more small conduits 76 along the flow path with not insubstantial back pressure. As the pressure in the cooling circuit drops, additional conduits are opened by further actuation of the depressurization valves 72 in stages, each stage opening a larger and/or more direct flow path between the coolant circuit 46 and the containment shell 54.
The initial depressurization stages couple a pressurizer tank 80 which is connected by conduits to the coolant circuit hot leg 56 and to spargers 74 in an in-containment refueling water supply tank 50. The spargers 74 comprise conduits leading to small jet orifices submerged in the tank, thus providing back pressure and allowing water to condense from steam emitted by the spargers into the tank 50. The successive depressurization stages have progressively larger conduit inner diameters. A final stage has a large conduit 84 that couples the hot leg directly into the containment shell 54, for example, at a main coolant loop compartment 40 through which the hot leg 56 of the reactor circuit 46 passes. This arrangement reduces the pressure in the coolant circuit expeditiously, substantially to atmospheric pressure, without sudden hydraulic loading of the respective reactor conduits. When the pressure is sufficiently low, water is added to the coolant circuit by gravity flow from the in-containment refueling water storage tank 50.
Automatic depressurization in the AP1000® reactor system is a passive safeguard which ensures that the reactor core is cooled even in the case of a major loss of coolant accident such as a large breach in the reactor coolant circuit. Inasmuch as the in-containment refueling water storage tank drains by gravity, no pumps are required. Draining the water into the bottom of the containment building where the reactor vessel is located, develops a fluid pressure head of water in the containment sufficient to force water into the depressurized coolant circuit without relying on active elements such as pumps. Once the coolant circuit is at atmospheric pressure and the containment is flooded, water continues to be forced into the reactor vessel, where the boiling of the water cools the nuclear fuel. Water in the form of steam escaping from the reactor coolant circuit is condensed on the inside walls of the containment shell, and drained back into the in-containment refueling water storage tank to be injected again into the reactor coolant circuit.
The AP1000® nuclear power plant has been designed such that in the event of a station blackout, i.e., the total loss of traditional onsite and offsite power, the plant can safely shut itself down and achieve a safe shutdown condition using only passive systems. By traditional onsite and offsite power, we are referring to electric power conventionally generated from onsite and offsite sources. A few simple valves align the passive safety systems when they are automatically actuated. In most cases, these valves are “fail safe.” They require power to stay in their normal, closed position. Loss of power causes them to open into their safe alignment. In all cases, their movement is made using stored energy from springs, compressed gas or batteries. The plant is designed to maintain this condition with no intervention for at least 72 hours after which some operator action is needed to extend the coping period. During the initial 72-hour period, battery banks are used to power any needed equipment and plant monitoring instrumentation, etc. It is desirable to explore additional passive means for extending this coping time beyond 72 hours by utilizing energy that is available within the plant at the time of and subsequent to shutdown.
Accordingly, it is an object of this invention to use the resources within the plant to safely maintain the plant beyond 72 hours without operator intervention or the assistance of offsite power.
It is a further object of this invention to so extend the coping period without altering the operation of existing plant systems.